Fiber Materials Feeding

The objective of fiber material feeding is to supply a predetermined and measured amount of fiber stock to the repulping unit. The feed is either continuous or batch-wise. The fiber material is delivered to the paper mill in the form of bales (virgin pulp, recovered paper) or as loose material (recovered paper). Only in the so-called integrated mills (pulp mill and paper mill integrated in one location) is the fiber material supplied to stock preparation by pumping the stock suspension directly from the pulp mill to the paper mill. An extra feeding system and repulping is only necessary for shutdown periods of the pulp mill.

Virgin pulp is delivered in bales (mostly sheeted material), which are bound together by wires into stack units of six or eight bales. The first step is to dewire the units by cutting the centrally arranged binding wires, to remove them automatically and to wind them to coils for easy disposal. The next step is to de-stack the unit into single bales thus preparing the bales for dewiring, i. e. to cut and remove the wires and wind them to coils as above. A metal detector may follow to detect any uncut wires which would then be cut and removed by hand. The wire removal efficiency is better than 96 % depending on the unit and bale quality. A further handling device may turn the bales through 180° for removal of the bottom packaging material. The bales are then fed to the repulping unit. In batch-wise working systems a weighing system will be installed. The capacity of such a virgin fiber material feeding line is up to 180 bales per hour. Figure 4.3 gives an overview of a handling system for virgin pulp units and bales. Figure 4.4 shows a closer view of a dewiring station for units and bales of virgin pulp.

 

Recovered paper is supplied to the mill in (individual) bales or as loose material. Often a mill has to handle both types. Automatic dewiring of recovered paper bales is more demanding than that of virgin pulp bales since these bales may vary in shape, size and kind of wiring. As in virgin fiber feeding systems the wires are cut and usually removed, the probability of bales not being dewired is less than 4 %, depending on the bale quality. Only in cases when raggers are installed in low consistency pulpers (mainly in board and packaging paper lines), are wires needed to build up the tail and to entangle plastic foils, strings, and textiles. On automatic cutting of the wires the bales open up extensively. After wire removal the bale structure is opened into loose material by a bale opener. Now the flow-stream of loose paper is equalized to the required height by a levelling drum. The levelled loose material on the conveyor belt is weighed by a radiometric weighing system. In combination with a conveyor speed control this gives a constant mass flow of fiber material into the repulping unit. The capacity of such a fiber material feeding line for recovered paper bales is up to 120 bales per hour depending on the quality of the recovered paper. In Figure 4.5 the schematic of a complete feeding system for baled and loose recovered paper is shown.

Disintegration

4.2.2.1 Repulping/Slushing

The purpose of repulping or slushing is to break down the dried primary fiber pulp or recovered paper into individual fibers or, at least, to form a suspension which can be pumped. In the latter case the remaining flakes have to be broken down in subsequent deflaking machinery. Repulping is needed not only at the beginning of the stock preparation system but also for the wet or dry broke from the paper machine.

During pulping the applied disintegrating forces have to be greater than the raw material strength. Wetting reduces the strength by breaking the fiber-to-fiber hydrogen bonds. Strength reduction by wetting is about 85–98 % for primary fiber pulp and nonwet strength recovered paper, and < 60–80 % for wet strength recovered paper. Recovered wet strength paper grades which are difficult to repulp may be slushed at elevated temperatures of more than 75 °C. Addition of chemicals – acidic or alkaline, depending on the wet strength agent – further assists wet strength reduction.

The relevant forces in repulping seem to result from viscosity, acceleration and mechanical clinging. Viscosity is mainly a matter of suspension consistency, together with velocity difference it creates shear stress. Acceleration of a particle results in inertia forces. Clinging of a flake e. g. around the rotor may induce viscosity, acceleration, or mechanical forces.

The steps in repulping are:

. • To feed the system with a predetermined rate of raw material and water

. • To wet the fibrous raw material rapidly and completely

. • To apply sufficient force to break the material down into individual fibers

• To discharge the suspension.

In the case of recovered paper repulping further steps may be necessary:

. • To remove solid contaminants such as foils, stickies, and printing ink from the fibers

. • To remove solid contaminants from the process at an early stage before they are broken down into too small particles which are difficult to remove in subsequent machinery

. • To mix process chemicals (such as deinking and bleaching agents) into the suspension

Depending on the raw material, the amount of production and the contaminants content, repulping is done in different types of pulpers or drums at consistencies between < 6 % and < 28 %. Slushing time is between about 5 and 40 min. Pulpers are usually stainless steel vats with a vertical axis. A concentric impeller is the slushing tool, vertical elements at the cylindrical wall and guide elements at the bottom redirect the rotating suspension flow to the vat center.

Low consistency (LC) pulpers (Fig. 4.6) comprise a flat impeller with circumferential speed of about 15–20 m s–1. They operate at consistencies of up to about 6 %. At the bottom they have a screening sieve with hole sizes of 6–20 mm for suspension extraction. Operation is either continuous for slushing of recovered paper (fluting and liner, high wet strength grades) and most of the primary fiber materials, or periodic for certain primary fiber applications. In recovered paper processing ongoing removal of trash has to be ensured in order to prevent excessive trash concentration which would reduce the production and quality and might even stop the pulper rotor. Figure 4.7 shows a LC pulper trash removal system. Part of the suspension in the pulper is extracted and fed to a junk separator to remove heavy contaminants. The following disk screen has two functions. It acts as a deflaker to reduce the number and size of the flakes and as a coarse screen for removal of remaining trash and oversized flakes. The reject is sorted in a drum screen, its accept being recirculated to the pulper and rejects being disposed. Often raggers are used for additional trash removal such as for bale wires, plastic, foils, and textiles.

 

Stock consistencies at medium consistency (MC) pulpers are up to about 12 %, those at high consistency (HC) pulpers up to about 19 %. Both pulper types have a helical rotor and usually no screen plate. Circumferential speed is about 12–17 m s–1. They generally operate intermittently and are used in processing recovered paper such as newspapers and magazines. Figure 4.8 shows an HC pulper and Fig. 4.9 an HC pulping system for recovered paper processing including dumping and dilution water feed system. For each batch unwired baled or loose raw material and water are fed to the pulper. After its reduction to the desired flake content and size at high consistency – and detachment of ink to a certain degree – dilution water is added. The suspension is then fed to a disk screen with deflaking and coarse screening functions. Its reject goes via a buffer tank to a drum screen, its accept to a dump chest. The reject of the drum screen leaves the system via a dewatering screw, the accept is recirculated.

 

Drum pulpers operate at consistencies of about 14 %-28 %. The drum is driven on the periphery, the axis is declined to the drum end. Drum pulpers are used in repulping of recovered paper of lower wet strength such as newspapers and magazines, fluting and liner as well as liquid packaging board. Due to the lower forces the size reduction of sensible contaminants such as stickies or thin foils is limited. Drum pulper systems combine the functions of slushing and coarse screening. There are two types on the market. One (Fig. 4.10) has a single drum body with a first zone for slushing and a second zone for coarse screening. The rotation speed is about 100–120 m min–1, the drum diameter 2.5–4 m, length up to 30–40 m, the slushing zone being about two thirds of the length. Slushing consistency is about 14–20 %, accept consistency of coarse screening (hole diameters about 6–9 mm) is about 3–5 %. As the drum rotates the raw material is lifted with the help of lifting baffles mounted in the axial direction. Disintegration occurs mainly by two principles: (i) During lifting part of the material rolls and slides back thus generating shear forces and (ii) the remaining part of the material lifted to higher position falls back to the pond. The resulting impact exerts effective slushing.

 

The second type distributes the two functions of slushing and of coarse screening between two individual drums, each operating at different circumferential speeds and consistencies adopted to the different functions (Fig. 4.11). Furthermore the slushing drum is equipped with a D-shaped “displacement core”. Both displacement core and drum are equipped with bars in the axial direction. The length of the slushing drum is about 7–15 m, that of the coarse screening drum 7–17 m. The rotation speed of the slushing drum is about 1.5 m s–1, that of the coarse screening drum about 2.5 m s–1. Consistency in the slushing part is about 23–28 %, that in the accept of coarse screening 3–5 %. The filling level of the slushing drum is adjusted to the actual production and is about 30–60 % of the drum volume. As the drum rotates the stock is exposed to shear forces in the up-going channel between the drum and the fixed displacement core. The impact of the stock falling down from the top further supports effective slushing.

 

Broke pulpers beneath the paper machine are found at the end of the wire section and the press section where the web is wet and easy to slush. Slushing in broke pulpers in the dryer section, at the size press or in coating stations needs more energy and time, as the web is dry. (Broke from other places outside the paper machine are treated in pulpers as described earlier.) Broke pulpers extend across the whole width of the paper machine and have to treat the full production. In the case of a web break, the broke pulper located upstream of the break position has to start its full operation almost immediately. Water showers direct the web into the pulper and provide the necessary amount of dilution water. A consistency of 3–5 % together with an optimized system of vat geometry and rotor ensure good stock circulation and slushing in the pulper. Circulation energy and defibering forces are exerted either by agitators with a horizontal axis and propellers mounted on these or by impellers such as found in pulpers in stock preparation. The disintegrated part of the pulper content is extracted from the pulper through a screen plate. Figure 4.12 shows an example of a broke pulper.

4.2.2.2 Deflaking

The objective of deflaking is to break down small pieces (flakes) of undisintegrated paper or pulp sheets into individual fibers. The residual flake content after the deflaker should be zero, in special cases at least below 5 %. Deflaking helps to avoid paper quality problems, to save fiber raw material and to ensure improved operating conditions for the succeeding machines in the process e. g. screening or cleaning. Deflaking is carried out in deflakers after slushing in a pulper or a drum, in the preparation of recovered paper, virgin pulp or broke. A remarkable deflaking effect also occurs in disk screens. Cylindrical screens or pumps have a lesser effect. Deflaking is done at stock consistencies of about 3 to 5 %. The shear forces necessary for disintegration are applied to the fiber bundles and flakes when they pass radially through the fillings slots of the intermeshing teeth of the rotor and the stator (Fig. 4.13). The peripheral speed of the rotor fillings is 25 to 40 m s–1. Depending on the entering flake size, flake content, and trash content, coarser or finer fillings for the deflaker are selected (Fig. 4.14).

For stocks with low deflaking resistance the specific energy demand is 20 to 40 kWh t–1. Deflaking is mostly done in a single pass. Two or more passes may be required for stocks which are more difficult to deflake. Flakes with high wet strength must be disintegrated in a disperser which can apply higher shear rates.

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4.2.3

Screening and Fractionation

4.2.3.1 Screening

The objective of screening is the removal of interfering solid substances from the suspension that differ from the fibers in size, shape and deformability. These can be solid nonpaper particles like plastics or paper flakes and fiber bundles. The suspension passes a screen with holes or slotted openings which are larger than the fibers but smaller than most of the particles to be removed. The latter are intended to be retained by the screen and extracted at the reject outlet together with a certain amount of fiber suspension. Clearing devices rotate at a small distance over the screen surface generating pressure pulses and thus prevent the sieve from plugging. These rotors should be not too aggressive in order to maintain lower strength nonpaper particles in a screenable size. The pressure difference across the screen may force deformable particles through the screen openings. Cleanliness efficiency for soft stickies, for instance, is therefore lower than for hard stickies of the same size. Increasing the pressure difference results in further loss of efficiency for soft stickies removal compared to that for hard stickies.

Screening is used in both primary and secondary fiber preparation. In the latter, screening is done at several positions in the system with different kinds of machines with different kinds and sizes of openings. Pre-screening (first coarse screening step) is integrated in the slushing systems followed by a (second) coarse screening step and fine screening so more and more trash is removed step by step, first the coarse and then the finer material. Thus the subsequent screening step can operate safely and with low abrasion, even at the high demands of fine screening.

Fiber loss from the reject of a screen is reduced by re-screening the first stage reject in a second, third or even fourth stage. The reject of the last screen (tailing screen) determines the fiber loss. A higher reject rate increases the cleanliness efficiency of a screening system but increases the fiber loss. (Cleanliness efficiency is the ratio of effective separation to the maximum separation theoretically possible). More stages in one screening system result in lower fiber loss but higher investment costs. So screening is always a compromise between machinery outlay, cleanliness efficiency, fiber loss, throughput and operating reliability.

4.2.3.1.1 Coarse Screening

In coarse screening both disk and cylindrical screens are used. As shown in Fig. 4.15 a disk screen consists of a conical housing, a screen plate, a vaned rotor, and baffle bars. Screen hole diameters are about 2–4-mm, the peripheral rotor speed is about 20–30 m s–1. Disk screens operate at consistencies below 6 %. Due to their effectiveness in flake defibering, disk screens are also used in the second screening stage of a system with a cylindrical screen in the first stage to reduce loss of paper flakes consisting of valuable fibers. Without deflaking the subsequent stages could not be operated reliably due to the enormous increase in flake content from stage to stage.

 

The design principle of cylindrical screens in coarse screening is usually identical or similar to that of fine screens. The exception is a machine type with rotating screen where the pulsing blades are stationary. This type of machine is only used in coarse screening. Cylindrical screens consist of a housing, a rotor with clearing devices and a cylindrical sieve (Fig. 4.16). Depending on the trash content of the suspension different types of rotors may be applied, one of is shown in Fig. 4.17. The defibering effect of cylindrical screens is lower than that of disk screens and depends on the rotor type. Cylindrical screens are used for suspensions with low flake content and operate at consistencies below about 5 %.

 

Final stage coarse screening machines must handle high trash content. On the other hand low fiber loss and high cleanliness efficiency are required. The machine in Fig. 4.18 is not pressurized and operates at a consistency of about 1–4 % in the accept. Screen hole sizes are about 2 to 4 mm. Rotors fitted with vanes keep the screen clear and transport the debris such as plastics to the outlet. During the pass through the machine water sprays support the separation of fibers and debris resulting in a low fiber content in the reject. Another type of tail screen (Fig. 4.19) uses in its lower section a pressurized disk screen whereas the upper part is a nonpressurized cylindrical screen, both usually equipped with holes. Pressurized cylindrical screens are also used in the last stage to give lower flake and debris content.

4.2.3.1.2 Fine Screening

For fine screening different types of cylindrical screens are used. They may differ in the geometry of the housing (for uniform basket throughflow), the rotor shape (influencing the pulse characteristics), the rotor position (at the entrance side of the suspension into the screen or at the backside) and its circumferential speed, the flow direction through the screen (centrifugal or centripetal), the kind and size of slots, and the calculated suspension velocity through the screen orifices as well as any special configuration to influence the suspension stream lines in the vicinity of the apertures. Fine screening is done at low stock consistency of below 1.5 %. The rotor is adjusted to the requirements of LC screening (Fig. 4.20), its circumferential speed is 10–30 m s–1.

The screen baskets are slotted with widths of 0.1–0.4 mm. Some baskets are milled, where the milling tool defines the slot width and the uniformity of width across the whole basket. Others are bar-type baskets (Fig. 4.21) where individual bars are affixed to mountings by welding, brazing or clamping. The distance between the bars is the slot width. The shape of the bars and the kind of mounting define a profile angle at the slot entrance. Both profile angle and slot width strongly affect the screening result: The smaller the slots and the lower the profile angle, the better the cleanliness effect, and vice versa. On the other hand finer slots and lower profile angle result in lower throughput and more thickening and fractionation. To maintain the slot geometry as long as possible during operation, abrasive particles should be removed from the suspension before screening. For that reason recovered paper processing systems mostly comprise LC cleaning ahead of LC screening.

Final stage screens in fine screening have slotted screen baskets due to quality requirements. Operation can be continuous or batch. Continuous operation of such a screen results in higher cleanliness but also higher fiber loss compared to batch operation. In batch operation flushing water washes out most of the fibers, but a larger amount of debris passes the screen during the wash cycle. Figure 4.22 shows a final stage screen where part of the reject is recirculated in order to prevent too high a thickening of the reject towards the outlet.

Each fine screening system comprises several stages, including a final stage screen. The individual screen accepts and rejects in the various stages are interconnected, depending on the requirements. Figure 4.23 shows feed forward, full or partial cascade operation schematics. In cascade systems a higher separation probability is given. Feed forward may be advantageous when debris particles tend to be easily comminuted in screens or pumps during cascading. An arrangement in series (A–B sequence) provides optimal cleanliness at low fiber loss. This sequence can be applied in the primary, intermediate or final stage.

4.2.3.2 Fractionation

Screening is defined as the separation of debris from the fibers in a suspension. As a principle the fiber fractions in the accept and reject should be the same as in the inlet. In contrast to that, fractionation aims to separating certain fractions of fibers, for instance long fibers from short fibers. The mass flow split in screening is about 5–25 %, in fractionation higher, 30–40 %, due to the production requirements.

Fractionation is done both in flat screens and in cylindrical screens as used for screening, with some differences as regards operating conditions and machine parts. The resulting separation of long and short fibers is far from complete, many long fibers are found in the short fiber fraction and vice versa, only a certain enrichment of long or short fibers is possible. Operating and machinery parameters that improve the fractionating effect are e. g. smaller openings (holes or slots), lower or zero profile angle of slotted screens, and higher consistency. For improved overall effect in some cases additional fractionation is done in cleaners.

With fractionation in screens the main debris flow goes with the stream enriched with long fibers. Here the debris has to be removed in a separate screening step.

4.2.4

Centrifugal Cleaning

The objective of centrifugal cleaning is to remove from the suspension particles that negatively affect paper quality or cause either excessive wear or plugging in subsequent processing machines. For their efficient removal the particle density must differ from that of water, and their size and shape from those of the desired suspension components. Centrifugal cleaning complements other separation methods like screening due to its different physical separation principle. In contrast to screening, hydrocyclone cleaning does not tend to deform softer particles.

Hydrocyclones are used in stock preparation of virgin pulp and of recovered paper where they are even more important. Different hydrocyclone types exist for operation at various consistencies depending on the location in the process, at high consistency HC (2–5 %) after slushing of the stock, or along the process line at medium consistency MC (up to 2 %) and at low consistency LC (0.5–1.5 %) at the end of stock preparation and in the approach flow system. HW ( heavy weight) cleaners may remove metal, glass, sand of particle sizes of about 10–100 mm up to 8–20 mm depending on the type, whereas LW (light weight) cleaners are effective for light particle removal such as wax or plastic foam of similar size range.

The separation takes place in the centrifugal field of so-called hydrocyclones (Fig. 4.24) that is generated by the velocity of the entering suspension. Here the heavy particles are forced to the outer wall (HW cleaners) whereas the light ones are driven to the center (LW cleaners). The flow streams where the heavy or light particles are accumulated are separated from the cleaned stock stream. The flow in a hydrocyclone is a three-dimensional two-phase flow. The circumferential component generates the centrifugal force, the axial component moves the solid particles towards the cleaner outlet and the radial component of the suspension flow proceeds from the outside towards the center and vice versa.

Hydrocylone efficiency generally increases with

• Increased centrifugal acceleration, obtained by high tangential velocity and the small diameter of the hydrocyclone. The velocity is dependent on the pressure differential between inlet and accept.

• Lower stock consistency, as the fiber network may restrict particle motion at elevated consistencies.

. • Appropriate reject removal without remixing of reject and accept streams.

. • Particles with

. – large density difference compared with water

. – large size at comparable density

. – favorable hydrodynamic shape (cw· A) at comparable density and size.

According to the flow directions of the reject and accept relative to the inlet hydro-cyclones are called either counterflow or unidirectional flow cleaners (Fig. 4.25). In counterflow inlet and accept or reject are at the top and reject or accept at the bottom. Unidirectional flow is defined by the reject and accept connections being opposite to the inlet.

Usually hydrocyclones have three connections (3-way with inlet, accept, reject of heavy or light weight particles). Some LC cleaners operate as 4-way cleaners (inlet, accept, reject of heavy and reject of lightweight particles) or even 5-way cleaners with an additional air outlet.

4.2.4.1 High Consistency (HC) Cleaners and Systems

HC cleaners are positioned after slushing and operate at consistencies of about 2–5 %, sometimes up to 6 %. They are the largest cyclones used in the paper industry. They have use for pre-cleaning to remove heavy particles of more than 1 mm in size. Their density has to be significantly higher than 1 g m–3. These particles may plug, wear or damage subsequent machinery and therefore they have to be removed. HC cleaners are built with or without a rotor and are mostly based on the counter flow principle. Reject discharge is batch-wise (mostly without a final stage) or continuous with a final stage reject discharge (Fig. 4.26). In the latter case the HC cleaner reject is diluted and then finally cleaned. This system is shown in Fig. 4.27.

4.2.4.2 Medium Consistency (MC) Cleaners

MC cleaners usually operate at consistencies of up to 2 %, they are medium sized and single stage cleaners with a junk trap. By removing glass, sand particles and paper clips the subsequent machinery is protected from wear and reliable operation is ensured.

4.2 Main Unit Processes and Equipment

 

4.2.4.3 Low Consistency (LC) Cleaners and Systems

LC cleaners operate at consistencies of 0.5 to 1.5 % and they are smaller than the HC or MC cleaners. HC cleaners are usually of the 3-way type. Due to the low consistency and high centrifugal forces (small diameter and high circumferential velocity) the removal effect is highest. On the other hand the energy demand per ton of stock is high due to the above operating parameters. Reject outlet is continuous.

By removing fine sand particles heavy weight cleaners protect the fine slotted baskets of the screens from wear and sand accumulation. During cleaning the suspension thickens and is highest near the wall. Here the heavy particles are also accumulated. To prevent the reject outlet from plugging and to reduce fiber content in the reject, dilution water is added at this position. The cleaner design must ensure flow conditions in the reject outlet area where the dilution water does not remix the separated heavy particles with the cleaned stock. Figure 4.28 shows details of the reject area of a HW LC cleaner.

Light weight LC cleaners today are recommended mainly for wax and plastic foam removal. They can also be advantageous when particles such as soft stickies may be deformed or reduced in size in a screen. A precondition is that their density should be lower than that of water.

A cleaner system consists of as many as four stages. The reject of the first stage is diluted and cleaned in the second stage, the accept is fed back to the inlet of stage one, the reject is diluted and cleaned in the third stage etc. This type of system configuration is called a cleaner cascade system (Fig. 4.29). Individual cleaners of one stage are connected to form cleaner batteries with a common distributor feeding the inlets of all cleaners of the stage. Their accepts and rejects flow to collector pipes (Fig. 4.30).

A special type of equipment is a centrifugal cleaner with rotating housing. Due to high centrifugal acceleration and the flow conditions, the cleaner can remove light particles with a density close to that of water.

Refining

The objective of refining or beating is to “design” the fibers to match the requirements of

. • the paper making process

. • the desired properties of the finished paper.

For example good dewatering characteristics of the stock are desired in the forming and press sections as well as a high potential for good formation quality in the forming section. Sufficient web strength is required for safe web transfer in the press and dryer sections when the paper web is still wet. Desirable paper properties may include certain strength properties (tensile, tear, burst, fold, Young’s modulus), bulk, air permeability, opacity or printability. During refining all stock and paper characteristics are more or less affected so optimization of the refining parameters has to ensure a sound compromise of the resulting stock and paper properties.

Refining is very important in the stock preparation process for virgin chemical pulp. For mechanical and recycled fibers refining has lower importance. The refining of recycled fibers usually aims for strength increase and shive elimination or reduction.

By refining, the shape of the fibers is changed. They may be shortened, split lengthwise, collapsed or fibrillated. Refining is done either at low consistency (virgin fibers, secondary fibers) of about 3–6 % or at high consistency (mainly secondary fibers) of about 30 % and more.

In refining the fibers pass between the bars of the fillings of the stator and rotor of a refiner. The operating parameters influencing the result in low consistency refining are

. • geometry (and material) of the fillings

. • net refining energy

. • specific edge load

In Fig. 4.31 a segment of a filling is shown. The angle between the bars of the rotor and the stator (cutting angle), the bar width and bar edge sharpness are the main influencing parameters of the fillings. As the bars are subjected to wear during operation the bar edges develop a shape that depends on the bar material and the load applied. The net refining energy is the amount of energy transferred to a specific amount of stock. It is the difference between total power consumption for the refining process and the “no-load” power when a defined volume flow of water or stock is pumped through the refiner at large spacing between the fillings. The refining energy transferred is controlled by the force pressing the stator and rotor together. The specific edge load is calculated from the net refining energy divided by the cutting edge length per second. The cutting edge length refers to the length formed per second by the edges of the bars as they move past each other.

Compared with fibrillation, cutting reduces the fiber length far more which results, for instance, in easier dewatering and helps to improve formation quality. On the other hand with cutting the strength potential of the fibers – especially tear

– will not be fully developed. Shortening of the fibers is more pronounced using fillings with a small cutting angle, small bar width and sharp bar edges. Furthermore, refining should be done under high specific edge load. Less energy is needed to increase the SR value of a stock.

In contrast, for fibrillation of the fibers the bars of the rotor and the stator should have a large angle, and the specific edge load should be low. The result is for instance better utilization of the strength potential of the furnish, but there is a negative influence on formation quality and dewatering. Fibrillating refining needs more energy to increase the SR value of a stock.

4.2 Main Unit Processes and Equipment

 

Today’s refiners for low consistency refining are double disk refiners, or refiners with conical or cylindrical geometry of the rotor/stator unit. Figure 4.32 shows a double disk refiner. Both sides of the rotor are equipped with fillings acting against the stator fillings of the front side (with loading device) and the backside (with drive). The front stator is moved by the loading device, the rotor can slide along the center shaft when loaded or unloaded. The peripheral speed of the rotor is about 25ms–1.

In Fig. 4.33 a refiner with cylindrical rotor/stator geometry is shown. The stock enters the center of the machine via the hollow center shaft and is refined during its helical horizontal path to the two stock outlet pipes. The refining energy is controlled by the gap between the rotor and the stator which is adjusted by cone-shaped means.

High consistency (30 % and more) refining is mainly based on the shear force effect between the fibers. This is why high consistency of the stock is required, which in turn necessitates its dewatering. To save costs HC refining is best placed in a system position where the stock is already dewatered for other purposes e. g. in order to separate the water loops (see Section 4.3).

HC refining preserves fiber length to a high degree, resulting in high dynamic strength properties, high elongation and porosity. HC refining is done either in special HC refiners or with disk dispergers such as described in Section 4.2.9. In this application the disk disperger is operated without steam heating.

Freeness (Canadian Standard Freeness CSF) or Schopper Riegler (SR value) are often used to check the effect of refining. Unfortunately this value can only partly characterize the actual properties of a stock. The properties of papers made from the same original stock but with a different kind of refining may vary in a wide range in spite of the same measured value of CSF or SR value. The same is true for the behavior of the stock in the paper machine. It needs additional measurements such as fiber length distribution, specific surface or flexibility of the fibers to obtain a better picture of a stock or refining process.

The refining energy required to increase the SR value is about 0.5 to 2kWh t–1 °SR–1. This value is influenced by the type of stock processed, the SR value and the refining conditions, as explained before. Some paper mills use a laboratory refiner (Fig. 4.34) to check the incoming fiber materials and to elaborate optimum refining conditions for their individual furnishes.

4.2.6

Flotation

4.2.6.1 Selective Flotation

Selective flotation is used in stock preparation systems for recovered paper processing. The objective is to remove contaminants from the suspension such as printing ink, stickies, fillers, coating pigments, and binders. In selective flotation air is injected into the suspension generating bubbles that are mixed with the suspension. Such an air bubble may catch one or more particles. The particles remain attached to the bubble and are carried to the surface of the suspension. The resulting foam containing the dirt particles is then withdrawn from the suspension. The selection criterion in flotation is the different surface wettability of the fibers to be retained and the particles to be removed. The surface of these particles is or has been rendered hydrophobic (water repellent). The size of the particles that can be removed at least reasonably effectively by selective flotation is limited to a range of about a minimum of 5 to10 mm up to a maximum of 250 to 500 mm.

“Deinking” of the stock is the main purpose of flotation in recovered paper processing: Removing ink particles increases brightness, removing dirt specks enhances cleanliness. Particles larger than 50 mm are usually called dirt specks and are visible with the naked eye. Depending on the recovered paper mixture and on the product demands, up to three flotation lines are installed in a system at different positions. The deinkability of the paper mixtures is different and depends e. g. on the paper grades, the printing process and the time after printing or on the water hardness in the mill. To optimize the mill’s operating parameters a laboratory flotation cell or a pilot flotation cell is often used. It is important that these test cells work on the same flotation principle as the actual system in the mill.

The main prerequisites for a good deinking result are

. • The particles have to move freely in the suspension: they have to be detached from the fibers.

. • The particles must have a floatable size and shape: larger particles have to be reduced in size, too small particles have to be agglomerated into larger ones, flat particles should be reshaped to cubic ones.

. • The particles should have sufficient hydrophobicity: if not given by nature to a sufficient extent this can be accentuated by applying surfactants to the suspension.

. • The air bubbles have to move freely: the consistency should not be too high

• A sufficient number of air bubbles of convenient size (in the 1 mm range) should be uniformly distributed in the suspension: effective bubble generation and mixing of bubbles and suspension have to be ensured.

In the past a large variety of flotation cell designs were used. All cells have to ensure bubble generation, collision of the ink particles with the bubbles, transport of the ink-bubble aggregate to the suspension surface, and foam removal.

. • Bubble generation: A simple way to generate bubbles is to press air through a permeable body such as perforated metal sheeting or ceramics. Here the bubble size depends mainly on the surface tension of the suspension, the air injection volume, the air injection openings and the suspension velocity at the openings. Dynamic mixers have rotating impellers with air outlets fed by compressed air. Static mixers make use of natural aspiration for air supply and of the kinetic energy for mixing. In this case bubble size is determined by the suspension properties and the energy involved. The relative air load (total air volume flow to total suspension volume flow) is mostly about 300 %, in some cases up to 1000 %.

. • Collision of the dirt particles with the bubbles: Collision is a matter of probability and can only happen by relative movement of bubble and particle. Using a permeable body for bubble generation needs a longer residual time – and thus ascending path length – of the bubbles in the suspension to increase collision probability. With dynamic mixers collision of bubbles and particles is intensified in the vicinity of the rotating mixing body. In static mixers the complete streams of both suspension and air pass through the mixing element and undergo an intensive mixing with high collision probability.

. • Transport of particle-bubble aggregate to the suspension surface: As soon as a particle has been attached to the bubble its detachment has to be avoided. Detachment may occur by too high shear forces due to turbulence, buoyancy and gravity. This suggests that one should make the ascending path length as short as possible. On the other hand a certain height of the suspension level is also advantageous: Collision probability is increased, especially important in designs where the collision probability at the moment of bubble generation is lower. It also often helps to still the suspension surface to enable effective foam removal and to avoid remixing of the foam with the suspension.

. • Foam removal: Foam discharge is done e. g. by free overflow over a weir, where a scraper may support the removal. In other designs the foam is discharged through sucking pipes by pressure difference, either by vacuum outside or pressure inside the cell.

. • Cell body: The cell body can be open but nowadays the cells are usually closed for environmental reasons. The closed cells can be pressurized or operate under a slight vacuum to avoid exhausts. One flotation line usually consists of several flotation steps where the accept stream of the preceding step is the inlet flow of the following one. The arrangement of these steps can also be different: individual cells for each flotation step connected by pipes to a complete line, individual cell compartments which are aligned in a horizontal or vertical direction in one overall cell body, and column-like cells with internal recirculation are all found. A special case is a vertical cyclone-shaped cell where the ascent of the bubbles is mainly due to the centrifugal forces and is directed to the center.

Figures 4.35–4.40 show schematic arrangements and photographs of three of the numerous different cell designs which also differ in the arrangements of the individual flotation steps. The circular cell in Fig. 4.35 and 4.36 is closed and pressurized. It is divided into 3–5 superimposed elementary cells. The suspension flows from top to bottom and is aerated after each elementary cell before being fed to the next lower one. Aeration is achieved by static mixers with self aspiration. The bubbles injected in each elementary cell ascend through the upper cells to the foam layer at the top. The foam is removed through pipes by pressure difference to the ambient. A reject valve allows control of the reject rate and composition in order to omit a secondary stage. Figures 4.37 and 4.38 show a flotation line with individual cells arranged in a horizontal circle. An air-dispersion rotor is used for aeration and mixing of air and suspension. The pressure difference between the inner aeration sector and the outer separation sector in the cell initiates the flow of the suspension from one cell to the next. The foam is removed over a weir to a common reject channel. The cell in Fig. 4.39 and 4.40 consists of an elliptical tube which contains individual cells arranged in line. In each cell the suspension is aerated by a self-aspirating static mixer working on the step diffuser principle. The foam flows over a weir and is collected in a common channel.

 

The foam from the flotation line contains – besides the particles to be removed – some fiber material, mainly short fibers and fines as well as fillers. This loss in valuable raw material has to be minimized. Reducing the amount of reject foam is limited as this negatively influences brightness and cleanliness. For quality and economic reasons a secondary flotation stage where the reject of the primary stage is floated to recover fibers, fines and some fillers, is in common use. The reject of the primary cells contains large quantities of air. Deaeration of the reject is often necessary to ensure stable operation of the secondary cells. This can be done e. g. in a deaeration cyclone including a mechanical foam breaker.

 

 

Typically, flotation lines operate at stock consistencies of 0.8–1.5 % and temperatures of 40–70 °C, neutral to slightly caustic suspension conditions (pH 7–9) and water hardness 5–30 °dH.

4.2.6.2 Nonselective Flotation (Dissolved Air Flotation DAF)

Nonselective flotation is used for process water clarification in the water loops (see Chapter 5). The objective is to dispose of all the undesired water components which cannot be removed by mechanical separation, such as anionic trash, fines, or microstickies. These components would negatively affect the production process and/or the product quality. As flotation is based on bubbles generated by de

pressurizing air-saturated water this unit process is called dissolved air flotation (DAF).

The different steps in nonselective flotation are:

. • Generating flocs: Flocculants (cationic polymers) are added and mixed with the water to be clarified. As a result the fine particles agglomerate to flocs. In addition coagulants can be used to transfer colloidal material (“anionic trash”) into microflocs in order to make it accessible to flocculants.

. • Bubble generation: First a side stream of the untreated water (sometimes also clarified water) is air-saturated in a tank at about 7 bar. The amount of air dissolved in the water is proportional to the pressure. With increasing temperature the amount of dissolved air is less. All remaining nondissolved air is removed as these bigger bubbles would negatively affect the further process. By depressurizing the water small air bubbles with a narrow size range are generated, finely distributed in the water.

. • Flotation: The side stream of the aerated water is mixed with the main stream of unclarified water and fed into a flotation tank. The fine air bubbles adhere to the flocs and rise to the surface where they form a stable layer of sludge. For good flotation results flow turbulences in the tank have to be kept to a minimum.

. • Sludge removal, clarified water outlet: The formed stable sludge is removed from the water surface and discharged. The clarified water exits from the bottom of the tank.

The largest equipment in dissolved air flotation is the clarification tank. It can be circular, rectangular, with or without built-in elements for flow guiding, and made from metal or concrete. The design depends e. g. on the requirements placed on the quality of the clarified water, the position in the process and the quantity of water to be clarified. Figures 4.41 and 4.42 show a schematic and a photograph of a

circular tank for dissolved air flotation. They show the central feed of the mixed streams of aerated and unclarified water, the sludge removal by a paddle, the discharge to the tank center by a feeding screw, and the extraction of the clarified water near the bottom at the periphery. The tanks can have diameters up to 25 m and throughputs of up to 2500 m3 h–1.

Bleaching of Secondary fibers

With bleaching in stock preparation systems the optical properties of secondary fibers are improved: the brightness of the stock is increased and a possible color shade is reduced.

There are two different bleaching principles (see Section 3.3):

. • Oxidative bleaching, mainly with peroxide as the bleaching agent, for brightness increase by fiber lightening.

. • Reductive bleaching, with either (sodium) dithionite or FAS (formamidine sulfinic acid) as the bleaching agent, for color value correction and brightness increase by color stripping and fiber lightening.

The main parameters influencing the bleaching result are the type of chemical, its dosage, pH value, temperature, and retention time.

Depending on the requirements of the finished stock either one or both bleaching types are integrated in a stock preparation system (see Section 4.3). The amount and type of bleaching agents have to be adjusted to the fiber composition of the stock and to the desired properties of the finished stock.

Peroxide bleaching is carried out in the presence of NaOH, sodium silicate, and sometimes chelating agents at elevated temperatures. The optimum dosage ratio of NaOH and peroxide prevents yellowing and makes best use of the peroxide. This ensures the best possible bleaching effect at the lowest chemical costs. This kind of bleaching is most effective at high stock consistencies of about 30 %. Hence a combination with the dispersion system which operates at high consistencies and temperatures is advisable, using the Disperger to admix the bleaching agents to the stock. Dosage of peroxide is about 1 to 2 % of 100 % active peroxide on oven dry pulp. The stock is then fed into a bleaching tower (downflow) to ensure sufficient retention time, about 30 to 60 min for wood-containing and 30 to 90 min for woodfree stock, at elevated temperatures of about 60 to 90 °C and a pH of 10 to 11. The brightness gain is about 4 to 11 %ISO for woodfree stocks and 2 to 5 %ISO for wood-containing stocks. These values depend strongly on the kind of raw material and the bleaching conditions. A system for peroxide bleaching is shown in Fig. 4.43.

Economic reductive bleaching of secondary fibers requires an oxygen-free stock as dithionite is sensitive to the oxygen contained in the air. Therefore the stock has to be deaerated which can be done sufficiently at a medium consistency of about 10 to 15 %. FAS is less sensitive, thus operating consistency can be as high as 30 % and a Disperger may be used for admixing of FAS. Best results are obtained when operating the Disperger under pressure and at temperatures up to 120 °C. Both agents are usually fed to the inlet of a medium consistency pump and admixed to the stock in the pump. Dosage is 0.4 to 1.0 % (dithionite) and 0.2 to 0.6 % (FAS). The chemical reaction is much faster than with oxidative bleaching and hence reductive bleaching can be carried out in a bleaching pipe or smaller tower (upflow) with a retention time of 15 to 60 min. The temperature is 60 °C (minimum) to 95 °C, pH 6.5 to 7.5 (dithionite) and 10.0 to 10.5 (FAS). A system for reductive bleaching is shown in Fig. 4.44.

Washing

Washing is applied in stock preparation systems for recovered paper processing. Here the objective of washing is to remove from the fiber suspension solid and/or dissolved substances which may negatively affect the papermaking process or the finished product quality. Dissolved and colloidal contaminants include e. g. organic and inorganic substances contributing to COD and anionic trash. Solid particles comprise fillers, coating and ink particles, microstickies and fines. Removal of ink particles by washing (wash deinking) has been quite usual in the United States. Washing is a filtrating-thickening process. To be washed out the particles have to be smaller than about 30 mm. The washing effect is the greater the smaller the particles, the lower the consistency at the washer entrance and the higher the outlet consistency of the thickened suspension mat. The maximum theoretical washing effect is given by the ratio filtrate flow/inlet flow. This theoretical number cannot be obtained in practice as more particles are retained in the fiber mat than corresponds to the above ratio due to a certain filtering effect.

The application of a washing unit and the kind and amount of substances to be washed out depends on the raw material, the other unit processes applied in the stock preparation system and the finished product requirements (see Section 4.3). A washing stage in a system requires effective cleaning of the filtrate, usually by nonselective flotation (see Section 4.2.6.2). This in turn means high solid loss from the system. If desired the washing effect and thus the solid loss can be adjusted

e. g. by cleaning only part of the filtrate or adjusting the washer itself if possible.

The machinery used for washing consists mainly of disk thickeners and high speed belt filters. Static filters such as inclined and curved screens, spray filters and pressure screen type washers are also found. Disk thickeners are described in Section 4.2.9. An example of a high speed belt filter is shown in Fig. 4.45. Each unit of this twin machine consists of an inlet feeding the suspension into the gap formed by the center roll and the wire. The filtrate is collected and the washed stock discharged. These machines operate at speeds of about 350 to 1000 m min–1. The suspension is dewatered under the pressure exerted by the wire tension, leaving the roll with a consistency of about 5 to 10 %. Inlet consistency is about 0.7 to

1.5 %. The washing effect can be adjusted over a wide range by the fiber mat thickness i.e by its basis weight. The higher the basis weight the lower the washing effect. The proportion of ash and fines in the filtrate also depends on the basis weight as larger particles (as fines are, on average, compared to fillers) are better retained than fillers when changing e. g. from very low basis weights of 20 to 40 or 50gm–2.

4.2.9

Dewatering

The objective of dewatering a fiber suspension in stock preparation is to separate the solids in the suspension from the water and dissolved ingredients. The reasons for dewatering the suspension are technological and economic. They are mainly

. • to separate the water loops as regards chemical and contaminant load as well as temperature

. • to adjust stock consistency to a defined level required by specific unit operations such as dispersing or bleaching

. • to recover fibers from the white or waste water

. • to increase consistency to the highest possible level when making wet laps or at discharge of the rejects.

Dewatering is a filtration process where a suspension stream of consistency ci is divided by a filter into a thickened part with a consistency of ct and a filtrate stream of consistency cf. At the start of filtering the retention of the solids on the filter is lowest. With time mat thickness increases, as does retention, which means that the solids content in the filtrate decreases. Dewatering usually aims for zero consistency in the filtrate, which in practice is not reached as retention is never 100 %. In contrast, washing takes advantage of this effect for solid/solid separation.

Various types of machines are used for dewatering in stock preparation: Drum filters, belt filters, twin wire presses, disk filters, and screw presses, as well as static filters such as inclined and curved screens. They can differ in several aspects such as

• the driving forces for dewatering e. g. gravity, vacuum, wire tension over a curved surface, mechanical pressing in a nip

. • the allowed inlet consistencies for ensuring safe operation, from 0.5 % up to more than 3.5 %.

. • the obtained outlet consistencies of a minimum 3 % up to more than 30 %

. • the filtrate consistency which may vary between ppm and % numbers.

Machinery selection is done according to technological and economic requirements.

4.2.9.1 Drum Thickeners

Drum thickeners (or slushers/deckers) comprise an open roll covered with a filter wire, rotating in a vat filled with suspension (Fig. 4.46). The feed consistency may vary between about 0.5 and 2.5 %. A slusher is used when the required discharge consistency is not more than about 3–4 %. For higher outlet consistencies of about 5–6 % a decker, which comprises an additional roll to press and further dewater the filter mat before discharge, is used. As the cylinder rotates a mat is built up on the outside of the filter drum. Here dewatering is governed by the differential head between the suspension level in the vat and the filtrate level inside the drum. In the case of a slusher the thickened stock overflows into a chute, in the case of a decker the mat is removed from the couch roll by a doctor blade. The filtrate in the drum is extracted from the inside through a hollow shaft.

4.2.9.2 Belt Filters, Twin Wire Presses

Here, in principle, the suspension is fed onto a horizontal moving wire and dewatered by gravity or additionally by a vacuum placed under the wire. Dewatering capacity can be further increased by a second wire covering the suspension and the bottom wire. By guiding this sandwich over roll(s) of different diameter(s) D under a tension S of the outer wire a dewatering pressure p is exerted: p = 2 S/D. The highest dry content of the thickened stock is obtained when additional dewatering is done in one or several press nips. Twin wire presses (Fig. 4.47) make use of all three principles and obtain high consistencies of about 25 to 50 %.

4.2.9.3 Disk Thickener

The filtering elements in disk thickeners are hollow disks covered on both sides with wires. Numerous disks are mounted closely and equally spaced on a hollow shaft. The length of the machine is up to about 12 m with 3.0–5.5 m disk diameter. The filter disks are immersed about halfway in the suspension in a trough. The driving force for dewatering is the head differential between the suspension height in the trough and the filtrate level. As the filter disks rotate (at 5–20 m min–1 at the circumference) a fiber mat builds up which continuously falls back to the trough due to gravity and flow forces. Thus the trough consistency increases and the thickened suspension exits over a weir. The filtrate is removed through the hollow shaft. Its consistency is high as the fiber mat is continuously removed resulting in low retention.

4.2.9.4 Disk Filters

Compared to disk thickeners disk filters (Fig. 4.48) additionally apply vacuum to further increase the consistency, production, and retention. They are used for white water cleaning in the paper machine water circuit (save-all) and for dewatering of pulp suspensions in pulping. Typical consistencies are 0.5–1.3 % at the inlet and up to 12–18 % at discharge. The filter disks are similar in design to those in disk thickeners. A filter mat builds up during rotation at the filter disk surface supported by the vacuum generated by a “barometric leg” governed by the differential head of filtrate level in the filter and the outlet to the ambiance. Due to the vacuum the mat remains fixed to the filter disks and is further dewatered after having emerged from the suspension (“drying zone”). The thickened stock is removed from the disks in the upper quadrant by knock-off showers and collected in a conveyor via chutes between the disks. The disks are then cleaned by oscillating cleaning showers before the filter area is re-immersed into the suspension, and filtration starts again. In the beginning filtration takes place by gravity and the filtrate consistency is high (cloudy filtrate) due to the thin fiber mat. Consistency decreases with increasing filter mat thickness (clear filtrate) when a vacuum is applied. The filtrate is divided into two streams by separating means in the filtrate zone, the cleaner one being used for shower water. Sometimes a third stream (superclear filtrate) is drawn off.

4.2.9.5 Screw Presses

Screw presses comprise a housing, perforated (round holes or slots) screens, and a rotating screw (Fig. 4.49). The stock is transported by the screw to the outlet, being dewatered en route. The free volume between the screw body and the housing, available during the transport, decreases in the axial direction by the increase in the dewatering pressure. Reduction in the free volume can be achieved by decreasing the screw pitch angle or screw rotor diameter and increasing the screw rotor shaft diameter. Since the filter mat is continuously removed from the screen cylinder a high consistency and dewatering capacity is possible. On the other hand the filtrate consistency is highest compared to all other dewatering methods. Screw presses are also for dewatering in reject handling systems.

 

4.2.10

Dispersion

Depending on the furnish quality as well as on the requirements of the finished product, the tasks of a dispersing system vary widely. They are e. g.

. • to reduce the size of dirt specks to below the limit of visibility

. • to reduce the size of stickies

. • to break down coating and sizing particles

. • to distribute wax finely

. • to detach ink or toner particles from fibers

. • to disintegrate fiber bundles

. • to treat fibers mechanically and thermally

. • to mix in bleaching agents

. • to decontaminate the stock as regards microorganisms.

Dispersing is used in secondary fiber processing. It is located at the point of water loop separation where the suspension is dewatered up to a consistency of about 25 to 35 %. Often bleaching is done in combination with dispersing. For high quality demands two dispersing steps may be applied in the process.

During dispersing high shear forces are applied to the debris particles to be dispersed and also to the fibers. The shear forces have to exceed the strength of these debris particles in order to reduce their size. Hence stock consistency must be about 24 to 30 % to ensure the transfer of the required amount of dispersing energy, and the temperature has to be elevated to reduce their strength.

A dispersing system consists of three major process steps (Fig. 4.50):

. • dewatering the suspension from 4 to 10 % up to the required consistency of about 22 to 35 %

. • heating the dewatered stock to a temperature of about 85–90 °C (at ambient pressure) or higher (up to 130 °C under pressure)

. • applying shear forces to disperse the stock.

Dewatering of the suspension is done in a screw press – in some systems in a twin wire press (see Section 4.2.9). The required stock temperature during dispersing is obtained by direct steam heating which may be done either in a separate heating screw or by steam fed directly into the disperser. Dispersing itself is done either in a high-speed disk disperger (Fig. 4.51) or in a slow-running kneading disperger (Fig. 4.52).

The fillings of a disk disperger (Fig. 4.53) usually have intermeshing teeth or sometimes refiner-type bars. The rotor/stator element spacing is 1 mm or less, peripheral speed is about 50 to 100 m s–1. The specific energy demand is 50 to 80 kWh t–1, in certain cases up to 150 kWh t–1. Disk dispergers are always operated at elevated temperatures otherwise the loss in freeness would be considerable. This effect may have use for HC refining of recycled fibers (see Section 4.2.5).

 

 

The fillings in a kneading disperger are much coarser than in a disk disperger. The rotor/stator spacing is about 10 mm, the peripheral speed is 5–15 m/s. The design may be 1-shaft or 2-shaft. Energy input for kneading dispergers is usually between 30 and 80 kW/t, in special cases up to 120 kW/t. They can be operated unheated at normal process temperatures without noticeably decreasing freeness of the stock.

Control of the transferred dispersing energy for the kneading disperger is by adjusting the entering stock consistency. For the disk disperger energy control is by adjusting the spacing of the rotor/stator elements.

4.2 Main Unit Processes and Equipment

The effects of the two disperger types in general are very similar with some differing tendencies:

. • the disk disperger may be advantageous when good sticky and dirt speck reduction is required

. • the kneading disperger is recommended when high porosity and bulk of the finished product are a priority.

Mixing and Storing

Mixing has to ensure that all individual component flows entering a mixing apparatus have been uniformly distributed in the exiting suspension flow. When mixing is done in a chest its whole volume has to be agitated continuously. This requires relatively high energy input.

Mixing of suspensions in chests is usually done at stock consistencies of 3 to 5 %. The geometry of mixing chests should have an approximate 1:1 up to 1:1.6 diameter to height ratio for minimum energy demand and good mixing effect. The suspension is agitated by a chest mixing propeller similar to a ship’s propeller as shown in Fig. 4.54. Specific energy demand is 0.2 to 0.5 kW m–3, depending on the size and geometry of the chest as well as on the stock type and consistency.

During storage of a suspension, demixing of water and fibers and other components may occur at low and medium consistencies. This has to be avoided. Stirring only a small part of the suspension at regular intervals at different positions in the storing chest is sufficient to hinder or correct separation and reflocculation. Thus the energy demand is limited.